As one of test methods for measuring rolling resistance of a tire, a force method described in JIS D4234 is well known. In this method, as illustrated in FIG. 6, a tire T is rotated by being pushed against an outer surface of a drum (a) that rotates at a predetermined speed V (e.g., 80 km/h in case of passenger car tires) with a predetermined vertical load (e.g., 80% of the maximum load capacity in case of passenger car tires). Then a tangential force Fx acting on the tire axle (b) is measured by a force component meter attached to the tire axle (b), and then the rolling resistance Fr is calculated based on the tangential force Fx.
Note that the tangential force Fx includes resistance of a bearing on the tire axle and parasitic loss of the air resistance on the tire T and the drum (a). Accordingly, a correction of the axial force Fx takes place to remove the parasitic loss. Here, the parasitic loss is measured by the skim test (e.g., JIS D4234) which is performed subsequent to the measurement of the tangential force Fx.
On the other hand, the force component meter amplifies a signal obtained from a sensor by an amplifier because the signal is very weak. At that time, however, “drift” in which an operating point of the amplifier shifts occurs, and it brings a problem of divergence between the actual value and the displayed value. Drift is a phenomenon that an output of the amplifier varies gradually so as to shift the zero point in the state without the addition of input signals to the two input terminals. The reason for this can be found such as variations in the temperature characteristics of transistors.
In the prior art, drift amount of an amplifier to the time was previously obtained through actual measurement and stored. A technique that corrects a value of the tangential force Fx measured by offsetting the drift amount based on the information has been proposed. Unfortunately, the drift amount is different by not only a rotational direction of the tire at the time of measurement but also each tire. Thus, it has been difficult to correct the drift amount accurately from the tangential force Fx, and therefore a fully satisfactory accuracy has not yet been achieved.
The following Patent Document 1 relates to the correction of the drift amount.